ERA21LA391 · Boeing E75

The pilot reported that, while the airplane was at an altitude of 3,500 ft, the engine lost total power. He attempted to regain engine power by adjusting the throttle, mixture control, fuel valve controls, and carburetor heat; however, the engine would not restart. As the pilot set up an approach to a nearby airstrip, he determined that the airplane would not reach the runway and performed a forced landing into trees, resulting in substantial damage to the fuselage and wings.

Postaccident examination of the engine revealed that the carburetor’s rotating mixture actuating rod was difficult to rotate, and binding occurred as the actuating rod was rotated by hand. Upon disassembly of the carburetor, the mixture control actuating rod and the metering needle, which is connected to the inner end of the rod, were found bent. No other damage to the carburetor was found. Because these components are largely or entirely internal to the carburetor, the bending of actuating rod and the metering needle was likely not a result of impact damage. The investigation was unable to determine how or when the carburetor components became bent.

Although the bends in the actuating rod and metering needle would not have prevented the mixture control from operating, the bends would have changed how the mixture setting in the carburetor responded to the mixture control lever in the cockpit. If the cockpit control was moved toward the cutoff position, the metering needle might not have performed as expected and might have moved into the cutoff position before or after the cockpit control reached its expected cutoff position. Likewise, with the cockpit control in the normal fullrich position, the metering needle might have been in a position that was either more or less than full rich. Therefore, the bent components likely resulted in a mixture setting that was not expected for any given cockpit mixture control setting. The investigation could not determine whether the mixture would have been too lean (less fuel) or too rich (more fuel), either of which could have resulted in the engine not producing power.

Four of the cylinders’ spark plugs had black-colored deposits on their electrodes, consistent with a rich mixture condition. Two spark plugs had gray-colored deposits, consistent with a normal mixture condition. The remaining three spark plugs were oil soaked, masking their electrode color, but these spark plugs would generally be more consistent with a rich mixture condition rather than a lean condition.

Verbatim NTSB analysis and probable cause from the NTSB dataset